UPDATE: Monday, Nov. 28, 2016—9to5Mac offers a method for seeing if your iPhone 6s is eligible for the program, by checking two digits of the serial number.

Apple has said that a few iPhone 6s smartphones are unexpectedly shutting down, confirming a problem reported last week by a Chinese consumer protection group.

The company said the problem is restricted to a small number of phones within a limited serial number range that were manufactured between September and October last year.

Apple said it was not a safety issue, and appeared to suggest that the problem would be resolved by a replacement battery which the company offered free. It did not say how many iPhone 6s phones were affected and in which markets.

Apple’s wireless carrier partners are not participating in the battery replacement program for the iPhone 6s, so eligible users have to contact Apple retail stores, authorized service providers or technical support. Users have been advised to back up their data to iTunes or iCloud, turn off their ‘Find My iPhone’ facility, and erase all content and settings in preparation for the battery change.

“If your iPhone 6s has any damage such as a cracked screen which impairs the replacement of the battery, that issue will need to be resolved prior to the battery replacement. In some cases, there may be a cost associated with the repair,” Apple said while announcing the program.

Users who believe their iPhone 6s phones was affected by the issue, and have already paid to replace the battery, can contact Apple about a refund, the company said.

To comment on this article and other Macworld content, visit our Facebook page or our Twitter feed.